CVD COATED POLYCRYSTALLINE c-BN CUTTING TOOLS

ABSTRACT

In one aspect, the present invention provides coated cutting tools comprising a PcBN substrate wherein a layer of single phase α-alumina is deposited by chemical vapor deposition directly on one or more surfaces of the substrate.

FIELD OF THE INVENTION

The present invention relates to cutting tools having coatings appliedby chemical vapor deposition (CVD) and, in particular, to CVD coatedpolycrystalline cubic boron nitride (PcBN) cutting tools.

BACKGROUND OF THE INVENTION

Cutting tools have been used in both coated and uncoated conditions formachining various metals and alloys. In order to increase cutting toolwear resistance and lifetime, one or more layers of refractory materialshave been applied to cutting tool surfaces. TiC, TiCN, TiOCN, TiN andAl₂O₃, for example, have been applied to cemented carbide substrates byCVD.

Al₂O₃ or alumina has been of particular interest as a coating forcutting tools. Alumina demonstrates various crystalline phases includingα, κ, γ, β AND θ with the α and κ phases occurring most frequently onCVD coated cemented carbide cutting tools. α-alumina has proven to be adesirable coating given its thermodynamic stability and general chemicalinertness at various temperatures encountered in cutting applications.However, deposition of α-alumina is increasingly difficult and oftensensitive to deposition conditions and the chemical identity of thecutting tool substrate. Prior literature, for example, has demonstratedthe requirement to sufficiently oxidize titanium carbide surfaces priorto alumina deposition in order to induce formation of the α-phase.Insufficient oxidation of TiC surfaces results in the production ofκ-alumina or a mixture of κ and α phases (see e.g., Vourinen, S., ThinSolid Films, 193/194 (1990) 536-546 and Layyous et al., Surface andCoatings Technology, 56 (1992) 89-95).

Moreover, α-alumina coatings on cemented carbide substrates havedisplayed significant adhesion problems leading to premature coatingfailure by delamination and other degradative pathways. Adhesionproblems in α-alumina coatings on cemented carbides has been attributedto significant interfacial porosity developed between the coating andsubstrate during α-alumina deposition (see e.g., Chatfield et al.,Journal de Physique, Colioque C5, supplement au n° 5, Tome 50, mai1989).

In view of these findings, α-alumina bonding layers have beenextensively researched and developed. The bonding layers are provided onsurfaces of cemented carbide substrates to ensure the nucleation andgrowth of α-alumina and to mitigate or eliminate the development ofinterfacial porosity (see e.g., Halvarsson et al., Surface and CoatingsTechnology, 68/69 (1994) 266-273 and Halvarsson et al., Surface andCoatings

Technology 76/77 (1995) 287-296).

Cutting tools based on polycrystalline cubic boron nitride (PcBN) arebecoming more popular given the high hardness and high thermal stability(up to about 980° C.) of cBN. PcBN cutting tools, for example, findapplication in machining case-hardened and through-hardened steels,superalloys, chilled cast iron and gray cast iron. Additionally, PcBNbased cutting tools are operable to run dry for clean machiningprocesses thereby saving coolant, maintenance and disposal costs.

Similar to cemented carbides, cutting tools based on PcBN substrates canalso benefit from the application of various refractory coatings. PcBNcutting tool substrates, for example, have been provided with TiCN,TiOCN, TiN and Al₂O₃ coatings. Nevertheless, as with cemented carbides,the deposition of α-alumina on PcBN cutting tool substrates occursthrough the use of one or more intervening layers over the substrate,including bonding or modification layers. U.S. Pat. No. 7,455,918 toGates et al. discloses PcBN cutting tools wherein α-alumina is depositedon modification layers residing between the PcBN substrate and theα-alumina layer.

SUMMARY

In view of the foregoing, the present invention, in one aspect, providescoated cutting tools comprising a PcBN substrate wherein a layer ofsingle phase α-alumina is deposited by chemical vapor depositiondirectly on one or more surfaces of the substrate. In some embodiments,a coated cutting tool described herein comprises a substrate comprisinggreater than 80 weight percent PcBN and a coating adhered to thesubstrate, the coating comprising a layer of single phase α-Al₂O₃deposited by chemical vapor deposition directly on a surface of thesubstrate. In some embodiments, the layer of single phase α-Al₂O₃deposited directly on a surface of the substrate comprising greater than80 weight percent PcBN has a critical load (L_(c)) of greater than 70 N.

Moreover, in some embodiments, a coating of a cutting tool describedherein further comprises one or more layers of MO_(x)O_(n)N_(z)deposited over the single phase α-Al₂O₃ layer, wherein M is a metalselected from the group consisting of metallic elements of Groups IVB,VB and VIB of the Periodic Table and x+y+z=1. In some embodiments, oneor more layers of MC_(x)O_(y)N_(z) are deposited over the single phaseα-Al₂O₃ layer by CVD or physical vapor deposition (PVD).

In another aspect, methods of producing coated cutting tools aredescribed herein. In some embodiments, a method of producing a coatedcutting tool comprises providing a cutting tool substrate comprisinggreater than 80 weight percent PcBN and depositing directly on a surfaceof the substrate by chemical vapor deposition a layer of single phaseα-Al₂O₃. In some embodiments, a method of producing a coated cuttingtool further comprises depositing a one or more layers ofMO_(x)O_(n)N_(z) over the single phase α-Al₂O₃ layer, wherein M is ametal selected from the group consisting of metallic elements of GroupsIVB, VB and VIB of the Periodic Table and x+y+z=1. In some embodiments,one or more layers of MO_(x)C_(y)N_(z) are deposited by CVD or PVD.

In another aspect, methods of cutting metal are described herein. Insome embodiments, a method of cutting metal comprises providing a metalwork piece and cutting the metal work piece with a coated cutting tool,the coated cutting tool comprising a substrate comprising greater than80 weight percent PcBN and a coating adhered to the substrate, thecoating comprising a layer of single phase α-Al₂O₃ deposited by chemicalvapor deposition directly on a surface of the substrate. In someembodiments, the coating of the cutting tool further comprises one ormore layers of MO_(x)C_(y)N_(z) deposited over the single phase α-Al₂O₃layer as described herein.

These and other embodiments are described in greater detail in thedetailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a substrate of a coated cutting tool according to oneembodiment described herein.

FIG. 2 illustrates a substrate of a coated cutting tool according to oneembodiment described herein.

FIG. 3 illustrates a substrate of a coated cutting tool according to oneembodiment described herein.

FIG. 4 illustrates an X-ray diffractogram of a coated cutting toolaccording to one embodiment described herein.

FIG. 5 illustrates a top down scanning electron micrograph (SEM) of asingle phase α-Al₂O₃ layer of a coated cutting tool according to oneembodiment described herein.

FIG. 6 illustrates an X-ray diffractogram of a coated cutting tool bodyaccording to one embodiment described herein.

FIG. 7 illustrates a top down SEM of a single phase α-Al₂O₃ layer of acoated cutting tool according to one embodiment described herein.

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by referenceto the following detailed description and examples and their previousand following descriptions. Elements, apparatus and methods describedherein, however, are not limited to the specific embodiments presentedin the detailed description and examples. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Numerous modifications and adaptations will bereadily apparent to those of skill in the art without departing from thespirit and scope of the invention.

In one aspect, the present invention provides coated cutting toolscomprising a PcBN substrate wherein a layer of single phase α-alumina isdeposited by chemical vapor deposition directly on one or more surfacesof the substrate. In some embodiments, a coated cutting tool describedherein comprises a substrate comprising greater than 80 weight percentPcBN and a coating adhered to the substrate, the coating comprising alayer of single phase α-Al₂O₃ deposited by chemical vapor depositiondirectly on a surface of the substrate.

Turning now to components of a coated cutting tool described herein, acoated cutting tool described herein comprises a PcBN substrate. PcBNsubstrates of coated cutting tools described herein comprise greaterthan 80 weight percent PcBN. In some embodiments, for example, asubstrate of a coated cutting tool comprises at least about 85 weightpercent PcBN. A substrate of a coated cutting tool, in some embodiments,comprises at least about 90 weight percent PcBN. In some embodiments, asubstrate comprises at least about 95 weight percent PcBN.

In some embodiments, a substrate of a coated cutting tool describedherein comprises PcBN in an amount ranging from greater than 80 weightpercent to about 97 weight percent. A substrate, in some embodiments,comprises PcBN in an amount ranging from about 85 weight percent toabout 95 weight percent.

In some embodiments, a substrate comprises a ceramic or metallic binderin addition to the PcBN. Suitable ceramic binders for use in a substrateof a coated cutting tool described herein, in some embodiments, comprisenitrides, carbonitrides, carbides and/or borides of titanium, tungsten,cobalt or aluminum. In some embodiments, for example, a substratecomprises a binder of AlN, AlB₂ or mixtures thereof. In someembodiments, a suitable metallic binder for PcBN substrates comprisescobalt. Moreover, in some embodiments, a substrate can comprise solidsolutions of any of the foregoing ceramic or metallic binders.

Compositional determination of a PcBN substrate described herein can beconducted by X-ray diffraction (XRD). The cutting tool substrate rakeface or flank face can be analyzed depending on cutting tool geometry.For compositional phase analysis of a PcBN substrate described herein,both a PANalytical Xpert MRD diffraction system fitted with a Eulereancradle and microfocus optics for PcBN compacts and tips, or aPANalytical Xpert MPD fitted with programmable optics for analysis of amonolithic solid piece of PcBN can be used.

Both x-ray diffraction systems are configured with focusing beam opticsand fitted with a copper x-ray tube and operating parameters of 45 KVand 40 MA. For analysis of the monolithic solid piece, the PANalyticalMRD is fitted with programmable divergence slit and programmableantiscatter slit. The x-ray beam width is controlled by an appropriatemask size and x-ray beam length is fixed at 2 mm using the programmableoptics. The PANalytical MPD is fitted with a linear strip solid statex-ray detector and nickel beta filter.

The PANalytical Xpert MRD system is configured with a microfocusmonocapillary optics of either 100μ or 500μ focal spot depending on sizeof PcBN compact. The PANalytical MRD is fitted with a linear strip solidstate x-ray detector and nickel beta filter.

Analysis scan range, counting times, and scan rate are selected toprovide optimal data for Rietveld analysis. A background profile isfitted and peak search is performed on the PcBN substrate diffractiondata to identify all peak positions and peak intensities. The peakposition and intensity data is used to identify the crystal phasecomposition of the PcBN substrate using any of the commerciallyavailable crystal phase databases.

Crystal structure data is input for each of the crystalline phasespresent in the substrate. Typical Rietveld refinement parameterssettings are:

-   -   Sample Geometry: Flat Plate    -   Linear Absorption Coefficient: Calculated from average specimen        composition    -   Weighting Scheme: Against lobs    -   Profile Function: Pseudo-Voight    -   Profile Base Width: Chosen per specimen    -   Least Squares Type: Newton-Raphson    -   Polarization Coefficient: 1.0        The Rietveld refinement typically includes:    -   Specimen Displacement: shift of specimen from x-ray alignment    -   Background profile selected to best describe the background        profile of the diffraction data    -   Scale Function: scale function of each phase    -   B overall: displacement parameter applied to all atoms in phase    -   Cell parameters: a, b, c and alpha, beta, and gamma    -   W parameter: describes peak FWHM

Any additional parameter to achieve an acceptable goodness of fit

PcBN substrates having compositional parameters described herein can beprovided in various constructions. In some embodiments, for example, acoated cutting tool comprises a monolithic solid piece PcBN substrate.In some embodiments, a PcBN substrate is provided as a compact or insertattached to a support by brazing or other joining technique. In someembodiments, a PcBN substrate is a full top cutting insert on a support.

FIG. 1 illustrates a monolithic solid piece PcBN substrate of a coatedcutting tool according to one embodiment described herein. The PcBNsubstrate (10) comprises a flank surface (12) and a rake surface (14),wherein the flank (12) and rake (14) surfaces intersect to provide acutting edge (16). The substrate also comprises an aperture (18)operable to secure the substrate (10) to a tool holder.

FIG. 2 illustrates a PcBN substrate of a coated cutting tool accordingto one embodiment wherein the PcBN substrate is provided as a compact orinsert joined to a support by brazing or other technique. As illustratedin FIG. 2, the cutting tool (20) comprises a support (22) having notches(24, 26) in opposing corners of the support (22). In some embodiments,the support (22) comprises a metal carbide, such as tungsten carbidewith a cobalt binder. A PcBN substrate (28) is provided as a compact orinsert which affixes by brazing or other technique within each of thenotches (24, 26). The PcBN substrate (28) has a rake surface (30) and atleast one flank surface (32). A cutting edge (34) is formed at thejuncture of the rake surface (30) and at least one flank surface (32).The cutting tool in the embodiment of FIG. 2 further comprises anaperture (36), which can assist the connection of the cutting tool (20)to a tool holder.

FIG. 3 illustrates a PcBN substrate of a coated cutting tool accordingto one embodiment wherein the PcBN substrate is provided as an insert onthe top surface of a support. As illustrated in FIG. 3, the cutting tool(50) comprises a support (52) having a top surface (54). In someembodiments, for example, the support (52) comprises a metal carbidesuch as a tungsten carbide with a cobalt binder. The PcBN substrate (58)couples to the top surface (54) of the support (52) by brazing or otherjoining technique. The PcBN substrate (58) comprises a rake surface (62)and at least one flank surface (64), wherein a cutting edge (66) isformed at the juncture of the rake surface (62) and at least one flanksurface (64).

As described herein, a coating adhered to the PcBN substrate comprises alayer of single phase α-Al₂O₃ deposited by chemical vapor depositiondirectly on the surface of the PcBN substrate. In being depositeddirectly on one or more surfaces of the PcBN substrate, the layer ofsingle phase α-Al₂O₃ does not reside on a modification layer, bondinglayer or any other intervening layer.

In some embodiments, the layer of single phase α-Al₂O₃ has a thicknessof at least about 2 μm. In some embodiments, the layer of single phaseα-Al₂O₃ has a thickness of at least about 5 μm. The layer of singlephase α-Al₂O₃, in some embodiments, has a thickness of at least about 10μm or at least about 15 μm.

In some embodiments, a layer of single phase α-Al₂O₃ deposited directlyon a surface of the PcBN substrate has a thickness ranging from about 2μm to about 20 μm. In some embodiments, the layer of single phaseα-Al₂O₃ has a thickness ranging from about 5 μm to about 15 μm. Thelayer of single phase α-Al₂O₃, in some embodiments, has a thicknessranging from about 3 μm to about 10 μm.

In some embodiments, the layer of single phase α-Al₂O₃ has an averagegrain size ranging from about 0.2 μm to about 5 μm. In some embodiments,the layer of single phase α-Al₂O₃ has an average grain size ranging fromabout 0.5 μm to about 2 μm. In some embodiments, the layer of singlephase α-Al₂O₃ has an average grain size ranging from about 1 μm to about3 μm. Grain size of a single phase α-Al₂O₃ layer described herein can bedetermined by taking a top down SEM image of the α-Al₂O₃ layer at amagnification of 5000×. A line is drawn on the SEM image in a randomdirection, and the grain size is calculated by the following formula:

Grain size=L/(N−1)

wherein L is the length of the line, and N is the number of grainboundaries intersected by the line. The foregoing measurement isrepeated for five (5) SEM images of the α-Al₂O₃ layer, and the resultinggrain size values are averaged to provide the average grain size.

In some embodiments, grains of the single phase α-Al₂O₃ layer display acolumnar structure. Moreover, in some embodiments, grains at the surfaceof the single phase α-Al₂O₃ layer display a polyhedral morphology.

In some embodiments, grains at the surface of the single phase α-Al₂O₃layer display a shape having an aspect ratio of at least 2 in a planeparallel to the surface of the α-Al₂O₃ layer. Aspect ratio, as usedherein, refers to the length of the grain in a plane parallel to thesurface of the α-Al₂O₃ layer divided by the width of the grain in aplane parallel to the surface of the α-Al₂O₃ layer. In some embodiments,grains at the surface of the α-Al₂O₃ layer have an aspect ratio of atleast 5 or at least 10. In some embodiments, grains at the surface ofthe α-Al₂O₃ layer have an aspect ratio ranging from about 2 to about 20or from about 5 to about 15. In some embodiments, grains at the surfaceof the α-Al₂O₃ layer have an aspect ratio ranging from about 2 to about10.

In some embodiments, grain morphology at a surface of the single phaseα-Al₂O₃ layer can be determined by taking a top down SEM of the surfaceat a magnification of 5000×. FIGS. 5 and 7 discussed further hereinprovide SEMs from which grain morphology at the surface of the singlephase α-Al₂O₃ layer can be discerned.

In some embodiments, a coating of a cutting tool described hereinfurther comprises one or more layers of MO_(x)O_(n)N_(z) deposited overthe single phase α-Al₂O₃ layer, wherein M is a metal selected from thegroup consisting of metallic elements of Groups IVB, VB and VIB of thePeriodic Table and x+y+z=1. In some embodiments, M is selected from thegroup consisting of Ti, Zr and Hf. In some embodiments, for example, alayer of TiN is deposited over the single phase α-Al₂O₃ layer. In someembodiments, a layer of TiCN or TiOCN is deposited over the single phaseα-Al₂O₃ layer. In some embodiments, one or more layers ofMO_(x)C_(y)N_(z) are deposited over the single phase α-Al₂O₃ layer byCVD or physical vapor deposition (PVD).

In some embodiments, one or more layers of MO_(x)O_(n)N_(z) can have anythickness not inconsistent with the objectives of the present invention.In some embodiments, a layer of MO_(x)C_(y)N_(z) has a thickness of atleast about 0.5 μm. In some embodiments, a layer of MO_(x)C_(y)N_(z) hasa thickness ranging from about 0.5 μm to about 5 μm or from about 1 μmto about 3 μm.

In some embodiments wherein one or more layers of MO_(x)O_(n)N_(z) aredeposited over the single phase α-Al₂O₃ layer, the MO_(x)C_(y)N_(z)layer(s) can be etched away to expose a surface of the single phaseα-Al₂O₃ layer for SEM analysis of grain morphology.

A coating comprising a single phase α-Al₂O₃ layer in direct contact witha PcBN cutting tool substrate described herein and an optionalMO_(x)C_(y)N_(z) layer deposited over the α-Al₂O₃ layer, in someembodiments, demonstrates a critical load (L_(c)) greater than 60 N orgreater than 65 N. In some embodiments, the coating has a critical load(L_(c)) greater than 70 N. L_(c) values for coatings recited herein weredetermined according to ASTM C1624-05—Standard Test for AdhesionStrength by Quantitative Single Point Scratch Testing wherein aprogressive loading rate of 10 N/mm was used.

In some embodiments, a coating comprising a single phase α-Al₂O₃ layerin direct contact with a PcBN cutting tool substrate described hereinand an optional MO_(x)C_(y)N_(z) layer deposited over the α-Al₂O₃ layerhas modulus (E) of at least about 390 GPa. In some embodiments, thecoating has a modulus of at least about 400 GPa. In some embodiments,the coating has a modulus ranging from about 380 GPa to about 420 GPa.

In some embodiments, a coating comprising a single phase α-Al₂O₃ layerin direct contact with a PcBN cutting tool substrate described hereinand an optional MO_(x)C_(y)N_(z) layer deposited over the α-Al₂O₃ layerhas a nanohardness of at least about 20 GPa. In some embodiments, thecoating has a nanohardness of at least about 24 GPa. The coating, insome embodiments, has a nanohardness ranging from about 15 GPa to about30 GPa or from about 20 GPa to about 25 GPa.

Coating modulus and nanohardness values recited herein were determinedfrom nano-indentation testing conducted with a Fischerscope HM2000 inaccordance with ISO standard 14577 using a Vickers indenter. Indentationdepth was set to 0.25 μm.

In another aspect, methods of producing coated cutting tools aredescribed herein. In some embodiments, a method of producing a coatedcutting tool comprises providing a cutting tool substrate comprisinggreater than 80 weight percent PcBN and depositing directly on a surfaceof the substrate by chemical vapor deposition a layer of single phaseα-Al₂O₃. In some embodiments, a method of producing a coated cuttingtool further comprises depositing a one or more layers ofMO_(x)C_(y)N_(z) over the single phase α-Al₂O₃ layer, wherein M is ametal selected from the group consisting of metallic elements of GroupsIVB, VB and VIB of the Periodic Table and x+y+z=1. In some embodiments,one or more layers of MO_(x)C_(y)N_(z) are deposited by CVD or PVD.

In some embodiments of methods described herein, a cutting toolsubstrate can comprise any PcBN content recited herein for thesubstrate. Moreover, in some embodiments of methods described herein,the single phase α-Al₂O₃ layer can comprise any of the compositional,chemical and/or physical properties recited hereinabove for a singlephase α-Al₂O₃ layer. Additionally, in some embodiments, one or morelayers of MO_(x)C_(y)N_(z) can comprise any of the compositional,chemical and/or physical properties recited hereinabove for aMO_(x)C_(y)N_(z) layer.

In some embodiments, depositing a layer of single phase α-Al₂O₃ directlyon a surface of the PcBN substrate comprises varying the rate at whichthe α-Al₂O₃ is deposited. In some embodiments, varying the rate at whichthe α-Al₂O₃ is deposited comprises transitioning from a lower rate ofα-Al₂O₃ deposition to a higher rate of α-Al₂O₃ deposition. In someembodiments, a higher rate of α-Al₂O₃ deposition is 1.5× greater than alower rate of α-Al₂O₃ deposition. In some embodiments, varying the rateat which the α-Al₂O₃ is deposited comprises transitioning from a higherrate of α-Al₂O₃ deposition to a lower rate of α-Al₂O₃ deposition.

In some embodiments, depositing a layer of single phase α-Al₂O₃ directlyon a surface of the PcBN substrate comprises varying the depositiontemperature of the α-Al₂O₃. Varying the deposition temperature, in someembodiments comprises transitioning from a higher α-Al₂O₃ depositiontemperature to a lower α-Al₂O₃ deposition temperature. In someembodiments, the difference between the higher α-Al₂O₃ depositiontemperature and the lower α-Al₂O₃ deposition temperature is at least 50°C. In some embodiments, the difference between the higher α-Al₂O₃deposition temperature and the lower α-Al₂O₃ deposition temperature isat least 130° C. In some embodiments, the difference between the higherα-Al₂O₃ deposition temperature and the lower α-Al₂O₃ depositiontemperature ranges from about 50° C. to about 150° C. In someembodiments, the difference between the higher α-Al₂O₃ depositiontemperature and the lower α-Al₂O₃ deposition temperature ranges fromabout 50° C. to about 130° C. In some embodiments, transitioning from ahigher α-Al₂O₃ deposition temperature to a lower α-Al₂O₃ depositiontemperature comprises transitioning from high temperature (HT) α-Al₂O₃deposition to medium temperature (MT) α-Al₂O₃ deposition.

Alternatively, in some embodiments, varying the deposition temperaturecomprises transitioning from a lower deposition temperature to a higherdeposition temperature.

In some depositing a layer of single phase α-Al₂O₃ directly on a surfaceof the PcBN substrate comprises varying the deposition rate anddeposition temperature of the α-Al₂O₃ as described hereinabove.

In another aspect, methods of cutting metal are described herein. Insome embodiments, a method of cutting metal comprises providing a metalwork piece and cutting the metal work piece with a coated cutting tool,the coated cutting tool comprising a substrate comprising greater than80 weight percent PcBN and a coating adhered to the substrate, thecoating comprising a layer of single phase α-Al₂O₃ deposited by chemicalvapor deposition directly on a surface of the substrate. In someembodiments, the coating of the coated cutting tool further comprisesone or more layers of MO_(x)C_(y)N_(z) deposited over the single phaseα-Al₂O₃ layer as described herein.

In some embodiments, metal cutting with a coated cutting tool describedherein is conducted dry, in the absence of any liquids and/orlubricants. In some embodiments, a metal work piece is selected from thegroup consisting of gray cast iron, case-hardened and through-hardenedsteels, superalloys and chilled cast iron.

These and other embodiments are further illustrated by the followingnon-limiting examples.

EXAMPLE 1 Coated Cutting Tool Body

A coated cutting tool described herein was produced by placing amonolithic solid piece substrate (ANSI Catalog No. SNM433S0820)comprising PcBN in an amount of 90 weight percent with the balanceAlN/AlB₂ binder in a CVD apparatus having an axial feed configuration. Alayer of single phase α-Al₂O₃ was deposited directly on a surface of thePcBN substrate according to the process parameters provided in Table I.

TABLE I CVD Deposition of α-Al₂O₃ Directly on PcBN Cutting ToolSubstrate Temp. Pressure Time H₂ AlCl₃ CO₂ HCl H₂S Process Step (° C.)(mbar) (min) (vol. %) (vol. %) (vol. %) (vol. %) (vol. %) Etching980-1000 60-200 10-50 balance 0.5-2.0 — — — Al₂O₃ 980-1000 60-200 10-50balance 0.1-1.0 0.2-1.2 0.2-1.0 — Deposition Al₂O₃ 980-1000 60-200  5-60balance 0.5-1.5 0.7-2.2 0.7-2.0 — Deposition Al₂O₃ 980-1000 60-200  5-60balance 0.5-1.5 0.7-2.2 0.7-2.0 0.01-0.08 Deposition Al₂O₃ 980-100060-200 120-600 balance 0.5-2.5 1.5-4.0 1.0-3.0 0.1-0.4 DepositionThe deposited single phase α-Al₂O₃ layer had a thickness of 7.2 μM. ATiN top layer of 1.5 μm was subsequently deposited over the single phaseα-Al₂O₃ layer by standard CVD techniques.

FIG. 4 illustrates an X-ray diffractogram of the coated cutting tool. Asprovided by the diffractogram, Al₂O₃ deposited directly on surfaces ofthe PcBN cutting tool substrate in the present Example was single phaseα-Al₂O₃. The α-Al₂O₃/TiN coating demonstrated an adhesion of greaterthan 70 N, a nanohardness of 24.2 GPa and a modulus of 406 GPa.

Moreover, FIG. 5 illustrates a top down scanning electron micrograph(SEM) of the coated cutting tool taken at a magnification of 3000×subsequent to removal of the top TiN layer by etching. Grains at thesurface of the single phase α-Al₂O₃ layer displayed a polyhedralmorphology.

EXAMPLE 2 Coated Cutting Tool Body

A coated cutting tool described herein was produced by placing amonolithic solid piece substrate (ANSI Catalog No. SNM433S0820)comprising PcBN in an amount of 90 weight percent with the balanceAlN/AlB₂ binder in a CVD apparatus having an axial feed configuration. Alayer of single phase α-Al₂O₃ was deposited directly on a surface of thePcBN substrate according to the process parameters provided in Table II.

TABLE II CVD Deposition of α-Al₂O₃ Directly on PcBN Cutting ToolSubstrate Temp. Pressure Time H₂ AlCl₃ CO₂ HCl H₂S Process Step (° C.)(mbar) (min) (vol. %) (vol. %) (vol. %) (vol. %) (vol. %) Etching980-1000 60-200 10-50 balance 0.5-2.0 — — — Al₂O₃ 980-1000 60-200 10-50balance 0.1-1.0 0.2-1.2 0.2-1.0 — Deposition Al₂O₃ 980-1000 60-200  5-60balance 0.5-1.5 0.7-2.2 0.7-2.0 — Deposition Al₂O₃ 980-1000 60-200  5-60balance 0.5-1.5 0.7-2.2 0.7-2.0 0.01-0.08 Deposition Al₂O₃ 870-89060-200  5-100 balance 0.7-2.0 1.0-2.5 1.0-3.5 0.1-0.4 Deposition Al₂O₃870-890 60-200 120-600 balance 1.0-3.0 4.0-10.0 1.0-3.5 0.1-0.4DepositionThe deposited single phase α-Al₂O₃ layer had a thickness of 9.9 μm. ATiN top layer of 0.6 μm was subsequently deposited over the single phaseα-Al₂O₃ layer by standard CVD techniques.

FIG. 6 illustrates an X-ray diffractogram of the coated cutting tool. Asprovided by the diffractogram, Al₂O₃ deposited directly on surfaces ofthe PcBN cutting tool substrate in the present Example was single phaseα-Al₂O₃. The α-Al₂O₃/TiN coating demonstrated an adhesion of greaterthan 70 N, a nanohardness of 26.2 GPa and a modulus of 390 GPa.

Moreover, FIG. 7 illustrates a top down scanning electron micrograph(SEM) of the coated cutting tool taken at a magnification of 5000×subsequent to removal of the top TiN layer by etching. Grains at thesurface of the single phase α-Al₂O₃ layer displayed an aspect ratioconsistent with those described hereinabove.

EXAMPLE 3 Cutting Tool Lifetime

Coated cutting tools described herein were subjected to cutting lifetimetesting in comparison with prior art coated cutting tools. Non-limitingembodiments of coated cutting tools of the present invention A, B and Cwere produced in accordance with Example 1 above. Non-limitingembodiments of coated cutting tools of the present invention D and Ewere prepared in accordance with Example 2 above. Compositionalparameters of coated cutting tools A-E and coated cutting tools of theprior art F, G and H are provided in Table III.

TABLE III Coating Compositional Parameters Coated Cutting Tool SubstrateCoating A PcBN (90 wt. %); AlN/AlB₂ α-Al₂O₃/TiN binder (balance) B PcBN(90 wt. %); AlN/AlB₂ α-Al₂O₃/TiN binder (balance) C PcBN (90 wt. %);AlN/AlB₂ α-Al₂O₃/TiN binder (balance) D PcBN (90 wt. %); AlN/AlB₂α-Al₂O₃/TiN binder (balance) E PcBN (90 wt. %); AlN/AlB₂ α-Al₂O₃/TiNbinder (balance) F PcBN (90 wt. %); AlN/AlB₂ Al₂O₃ (0.5 μm)/TiN binder(balance) (0.5 μm)/TiCN (1.5 μm)/Al₂O₃ (4 μm)/TiCN—TiN (2 μm) G PcBN (90wt. %); AlN/AlB₂ Al₂O₃ (0.5 μm)/TiN binder (balance) (0.5 μm)/TiCN (1.5μm)/Al₂O₃ (4 μm)/TiCN—TiN (2 μm) H PcBN (90 wt. %); AlN/AlB₂ Al₂O₃ (0.5μm)/TiN binder (balance) (0.5 μm)/TiCN (1.5 μm)/Al₂O₃ (4 μm)/TiCN—TiN (2μm)Coated cutting tools A and F were subjected to cutting lifetime testingas follows:

-   Work piece—Class 40 G2 gray cast iron-   Cutting Speed—2000 sfm-   Feed Rate—0.009 ipr-   Depth of Cut—0.025 inch-   DRY    Coated cutting tools B, D and G were subjected to cutting lifetime    testing as follows:-   Work piece—Class 40 G2 gray cast iron-   Cutting Speed—3000 sfm-   Feed Rate—0.009 ipr-   Depth of Cut—0.025 inch-   DRY    Coated cutting tools C, E and H were subjected to cutting lifetime    testing as follows:-   Work piece—Class 40 G2 gray cast iron-   Cutting Speed—3500 sfm-   Feed Rate—0.009 ipr-   Depth of Cut—0.025 inch-   DRY    The results of the cutting tool lifetime testing are provided in    Table IV.

TABLE IV Coated Cutting Tool Lifetime Results Cutting Speed CoatedCutting Tool Lifetime (minutes) 2000 sfm A 14.2 2000 sfm F* 12.1 3000sfm B 8.6 3000 sfm D 9.2 3000 sfm G* 6.7 3500 sfm C 5.9 3500 sfm E 7.23500 sfm H* 2.6 *Prior Art Cutting Tools

As provided in Table IV, cutting tools having an architecture describedherein (A-E) demonstrated significant increases in lifetime at allcutting speeds in comparison with the prior art coated cutting tools(F-H). Coated cutting tool E of the present invention, for example,displayed a 177% increase in cutting lifetime at a cutting speed of 3500sfm in comparison with prior art cutting tool H.

Various embodiments of the invention have been described in fulfillmentof the various objects of the invention. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Numerous modifications and adaptations thereof willbe readily apparent to those skilled in the art without departing fromthe spirit and scope of the invention.

1. A coated cutting tool comprising: a substrate comprising greater than80% weight percent polycrystalline cubic boron nitride (PcBN); and acoating adhered to the substrate, the coating comprising a layer ofsingle phase α-Al₂O₃ deposited by chemical vapor deposition directly ona surface of the substrate.
 2. The coated cutting tool of claim 1,wherein the substrate comprises at least 90 weight percent PcBN.
 3. Thecoated cutting tool of claim 1, wherein the layer of single phaseα-Al₂O₃ has a thickness of at least about 2 μm.
 4. The coated cuttingtool of claim 1, wherein the layer of single phase α-Al₂O₃ has athickness ranging from about 2 μm to about 20 μM.
 5. The coated cuttingtool of claim 1, wherein the layer of single phase α-Al₂O₃ has anaverage grain size ranging from about 0.2 μm to about 5 μm.
 6. Thecoated cutting tool of claim 1, wherein the layer of single phaseα-Al₂O₃ has an average grain size ranging from about 0.5 μm to about 2μm.
 7. The coated cutting tool of claim 1, wherein grains at the surfaceof the single phase α-Al₂O₃ layer display a polyhedral morphology. 8.The coated cutting tool of claim 1, wherein grains at the surface of thesingle phase α-Al₂O₃ layer have an aspect ratio greater than about 2 ina plane parallel to the surface.
 9. The coated cutting tool of claim 1,wherein the coating has a critical load (L_(c)) greater than 65 N. 10.The coated cutting tool of claim 1, wherein the coating has a criticalload (L_(c)) greater than 70 N.
 11. The coated cutting tool of claim 1,wherein the coating further comprises a layer of MO_(x)O_(n)N_(z)deposited by chemical vapor deposition over the single phase α-Al₂O₃layer, wherein M is a metal selected from the group consisting ofmetallic elements of Groups IVB, VB and VIB of the Periodic Table andx+y+z=1.
 12. The coated cutting tool of claim 1, wherein the coating hasa nanohardness of at least about 23 GPa.
 13. The coated cutting tool ofclaim 1, wherein the coating has a modulus of at least about 380 GPa.14. A method of making a coated cutting tool comprising: providing asubstrate comprising greater than 80% weight percent polycrystallinecubic boron nitride; and depositing directly on a surface of thesubstrate by chemical vapor deposition a layer of single phase α-Al₂O₃.15. The method of claim 14, wherein the substrate comprises at least 90weight percent PcBN.
 16. The method of claim 14, wherein depositingcomprises varying the rate of deposition of the α-Al₂O₃.
 17. The methodof claim 16, wherein varying the rate of deposition of the α-Al₂O₃comprises transitioning from a lower rate of α-Al₂O₃ deposition to ahigher rate of α-Al₂O₃ deposition.
 18. The method of claim 14, whereindepositing comprises varying deposition temperature of the α-Al₂O₃. 19.The method of claim 18, wherein varying the deposition temperaturecomprises transitioning from a higher deposition temperature of theα-Al₂O₃ to a lower deposition temperature of the α-Al₂O₃.
 20. The methodof claim 14, wherein depositing comprises varying the rate of depositionof the α-Al₂O₃ and varying deposition temperature of the α-Al₂O₃.